Adds a new operator to the pose slider tools that blends the
current pose with the neighbouring poses in the timeline.
The operator can be called in pose mode with Shift+Alt+E
or from the "pose" menu under "In betweens/Blend to Neighbour"
Reviewed by: Sybren A. Stüvel
Differential Revision: https://developer.blender.org/D9137#inline-105214
Ref: D9137